This invention relates to heat transfer through an article wall. More particularly, it relates to an article, for example a turbine engine component, having a first wall surface exposed to or affected by a fluid at a first temperature, for example the flowstream of the engine. The first wall surface substantially is opposite a second wall surface desired to be reduced, for example by cooling air, by heat transfer through the wall to a second temperature lower than the first temperature.
Components of a gas turbine engine, for example in the turbine section of the engine, operate in or are exposed or subjected to a heated stream of gas and/or products of combustion. Typical components include stationary shrouds or shroud segments and their supporting structure, blading members such as stationary or rotating airfoils of blades, vanes or struts, as well as walls of internal fluid flow passages, generally downstream of the combustor section of the engine.
Engine designers desire to operate a gas turbine engine at relatively high temperatures most efficient for the materials used in generating and extracting energy from the fluid produced. However, such elevated temperatures frequently are higher than those at which the materials from which components are constructed can withstand alone without compromise of mechanical and/or physical properties, or distortion of such materials. Consequently, use has been made of various coatings and/or of convection cooling air impinging on or flowing about and/or within a component to lower a wall temperature and reduce the damaging effect of excessive heat.
The heat transfer rate from a convection-cooled surface, to reduce its surface temperature, is linearly proportional to the surface area and its surface heat transfer coefficient. Accordingly, there has been reported a variety of surface configurations to increase such surface area. For example, in U.S. Pat. No. 5,353,865 xe2x80x94Adiutori et al. patented Oct. 11, 1994), a surface has been textured with a plurality of spaced apart protuberances such as dimples or ridges to increase the surface area for enhanced heat transfer therefrom, reducing the surface temperature. Another example of wall ridges and grooves to increase surface area for heat transfer enhancement is shown in U.S. Pat. No. 6,142,734xe2x80x94Lee (patented Nov. 7, 2000). In general, these kinds of structures sometimes are referred to a turbulators. The effects of turbulators are more extensively discussed in xe2x80x9cEffects of Turbulator Profile and Spacing on Heat Transfer and Friction in a Channelxe2x80x9d by Taslim and Spring, (Journal of Thermophysics and Heat Transfer, Vol. 8, No. 3, July-September 1994).
Although the above general type of surface protuberances can enhance heat transfer from a surface, manufacture of such structures as dimples of a size most desirable for heat transfer, typically by precision casting, has resulted in relatively low casting yields. For example, it is desired for enhanced heat transfer to cast such dimples with a diameter and height in the range of about 0.025-0.040xe2x80x3. However, such small dimple size has resulted in poor casting yield, for example because of problems relating to poor filling of casting mold cavities. Grooves or ribs of the generally continuous type described in the above identified Lee patent, generally continuous in the longitudinal direction, are easier to cast in a smaller size. However, when cooling fluid flows generally in parallel along the continuous groove or rib length, an insulating boundary layer of cooling fluid grows or extends along such a surface resulting in a poorer heat transfer coefficient. It is desirable to have a protruding elongated rib type surface configuration, preferably of a relatively small dimensional size, that has heat transfer enhancement as a result of disruption of such an insulating boundary layer. In addition, a rib with a relatively small size cross sectional dimension can be precision cast with a relatively high yield rate.
The present invention, in one form, comprises a wall having a first wall surface subject to a first temperature, and a substantially opposite second wall surface for exposure to a cooling fluid for reducing the second wall surface to a second temperature less than the first temperature. The second wall surface includes a plurality of spaced apart ribs protruding from the second wall surface to facilitate transfer of heat from the second wall surface. To enhance heat transfer from the ribs by disrupting boundary layer insulation problems and yet enable a good casting yield, the ribs are interrupted along their length. The ribs included on the second wall surface comprise a plurality of elongated rib portions, each portion spaced apart one from another across a gap between the rib portions having a gap length in the range of about 0.002-0.05xe2x80x3. In the precision cast form of an article, the fib portions have a cross section width dimension in the range of about 0.002-0.05xe2x80x3 and a rib portion length of at least about ten times the cross sectional width dimension of the rib portion.